I found, during times of high humidity and air temps, especially with a newer engine, the high speed can be pretty tricky to adjust. I've been messing with small engines most of my life and generally don't find tuning them difficult at all. I can get it running OK by ear, but once there, I have to depend on reading the plug to see what's really going on! A couple of minutes at wide open throttle in the air, then killing the engine and landing to get a good plug reading might be a good plan?

There's also the possibility you may be running into cooling issues?